1. Field of the Invention
The present invention is related, in general, to a method and apparatus for the treatment of sleep disordered breathing and, more specifically, to a method and apparatus for stimulating the hypoglossal nerve of a patient for the treatment of sleep disordered breathing.
2. Description of the Related Art
Sleep disordered breathing encompasses a number of illnesses including snoring, upper airway resistance syndrome (UARS) and obstructive sleep apnea-hypopnea syndrome (OSAHS). Snoring occurs when there is an obstruction in the back of the mouth which causes these structures to vibrate during breathing. Upper airway resistance syndrome is a disorder that results in an increased resistance to airflow. Obstructive sleep apnea (OSA) results in the partial or complete occlusion of the upper airways of human patients during sleep. In these patients, the upper airways obstruct as often as several times a minute with each episode lasting as long as 20-30 seconds. Each apneic episode ends with a brief arousal from sleep. Consequently, arterial oxyhemoglobin saturation decreases drastically. Complications include excessive daytime sleepiness, restless sleep, morning headache, job-related accidents, impaired short-term memory, polycythema, hypertension, right-sided congestive heart failure, decreased libido, and the like. Personality disorder and other psychological problems may also develop over time. Obstructive sleep apnea is found in 2 to 4 percent of the population, primarily in adult men and post-menopausal women.
In humans, the hypoglossal nerve innervates the intrinsic and extrinsic muscles of the tongue and the geniohyoid muscle. Of these muscles innervated by the hypoglossal nerve, the genioglossus and the geniohyoid muscles are the primary muscles involved in dilating the upper airways (UAWS). Contraction of the genioglossus muscle provides tongue protrusion and, hence, dilates the airways.
It is generally known that the flow of inspired air is doubled by stimulation of the main branch of the hypoglossal nerve. Stimulation of the medial branch is nearly as effective and is superior to stimulation of other branches. Attempts have been made to improve upper airway patency in humans during sleep via direct electrical stimulation of the hypoglossal nerve. For instance, U.S. Pat. No. 4,830,008 to Meer discloses monitoring inspiratory effort followed by stimulation of nerves associated with stimulating the upper airway muscles to treat sleep apnea. The Meer system includes an airway monitor for monitoring inspiratory effort based on sensing “action potentials” in either the diaphragm or phrenic nerve or by sensing negative pressure in the thorax and one or more “effector” electrodes that stimulates the selected upper respiratory muscle nerves.
U.S. Pat. No. 6,587,725 to Durand et al. discloses a method of treating obstructive sleep apnea in a human patient. The method includes the steps of monitoring the human patient for at least partial occlusion of upper airways of the patient associated with obstructive sleep apnea by sensing electroneurogram activity of a hypoglossal nerve of the patient; and, directly electrically stimulating the hypoglossal nerve of the patient when at least partial occlusion of the upper airways of the patient occurs as indicated by the sensed electroneurogram activity of the hypoglossal nerve. A limitation of this method is that it only stimulates the hypoglossal nerve if at least a partial occlusion of the upper airways of the patient occurs. Therefore, this method does not prevent occlusions from occurring.
Other methods of stimulating the hypoglossal nerve include using various physiological variables to synchronize the electrical stimulation with respiration. Hypopharyngeal or espophageal pressure measurements, airflow measurements made with thermistors placed near the nose and mouth, and tracheal inter-ring distance measurements made with a strain gauge are examples of physiological variables that have been investigated for use in synchronization of electrical stimulation of the hypoglossal nerve and/or the genioglossus muscle. For instance, U.S. Pat. No. 5,211,173 to Kallok et al. discloses a method and apparatus for controlling one or more parameters of an electrical stimulation generator used for the treatment of obstructive sleep apnea. Sensors are used to determine the effectiveness of the stimulation. Amplitude and pulse width are modified in response to the measurements from the sensors. However, the method disclosed in this patent and other similar methods have drawbacks and limitations.
Other treatment methods for sleep disordered breathing have included use of a nose mask through which continuous positive airway pressure is applied to keep the upper airways open. This therapy must be continued indefinitely, and only 60-65 percent of these patients can tolerate the technique long-term. Tracheostomy is another treatment for severe sleep disordered breathing, but it is rarely used because of low patient acceptability and relatively high morbidity. Uvulopalatopharyngoplasty, removal of redundant tissue in the oropharynx, and other surgical operations to correct anatomical abnormalities in the upper airways can be considered in certain cases. However, in general, all of the above-mentioned therapies are associated with complications and disadvantages. Weight loss may improve the condition in mild cases, but pharmacologic attempts to treat sleep disordered breathing by increasing pharyngeal muscle activity during sleep have not been found to be effective.